The present invention relates to the transmission of digital speech signals at reduced bit rates, and more particularly is related to a technique known as "nearly instantaneous companding" (NIC).
Still more particularly, the present invention relates to a gain control device which may be provided at the input to such a NIC processor.
In a conventional NIC processor, speech amplitude samples are companded according to the well known A-law or .mu.-law algorithms, and the digitally companded samples are then provided to a sample memory. A quantizer receives the samples from the memory and quantizes these samples, and the quantized samples are then encoded for transmission. A maximum sample finder determines the maximum value of N digitally companded speech samples within the memory, and provides a control signal to the quantizer to control the quantizing step size in accordance with the maximum sample value. By varying the step size with a fixed number of quantization levels, the quantizer can cover a wide range of signal amplitude values with a smaller transmission bit rate requirement. The encoder also receives the output from the maximum sample finder so that this information can be transmitted together with the quantized speech signals for use at the receiver in decoding.
A problem with such a conventional NIC processor is that it always utilizes the same number of quantization levels regardless of the maximum sample value, and therefore must vary its quantization range by varying the step size. At large speech amplitudes, the step size increases and this results in an increase in the quantizing noise. To overcome this problem, the present inventors have developed an improved NIC processor which varies the number of quantizing levels as a function of the maximum sample value S.sub.max. This improved NIC processor is described in copending and commonly assigned application Ser. No. 218,976 entitled "Nearly Instantaneous Companding Processor Using Variable Precission Block Quantization", the entire disclosure of which is incorporated herein by reference. The improved NIC processor described in application Ser. No. 218,976 is advantageous in that not only the quantization step size but also the number of quantization steps are varied for each block of samples in accordance with the value S.sub.max to thereby improve the overall signal-to-noise (SNR) ratio. However, the improved processor still suffers from the disadvantage that it yields a higher SNR for high levels of speech, e.g. levels above approximately -15 dBmO, than that achieved for lower level signals. Further, as the speech signal amplitude decreases, the number of quantizing steps will decrease to thereby result in a drop in the average transmission bit rate. It would be desirable to maintain a constant average bit rate of approximately 32 kbit/s, but the actual transmission rate may vary as much as 25 percent from this due to variations in signal amplitude.